HOIST FOR VEHICLES

Abstract: A hoist for goods vehicles of the type having a frame (2) and a load carrier (1) comprises a vertically adjustable platform (4), a pivotable lifting arm (5) which at spaced-apart articulation points (7, 8) is articulated to the frame and the platform, respectively, at least one linear motor (6) by means of which the lifting arm is pivotable whilst moving the platform between upper and lower end positions, and a parallel-motion mechanism (15, 16) for maintaining the platform at an essentially constant angle to the horizontal plane when it is being raised or lowered. The parallel-motion mechanism comprises a lever (16) and a parallel-motion arm (15), which are interconnected by an articulation (20). The lever (16) is further articulated to the lifting arm (5) by an articulation (18) and to the platform (4) or the frame (2) by an articulation (19). The parallel-motion arm (15) is, by an articulation (21), articulated to the one of the frame and the platform that is not connected to the lever. Furthermore, at least one of the articulations (7, 8, 19, 21) consists of a displaceable articulation bearing.

Description: HOIST FOR VEHICLES This invention relates to a hoist for vehicles of the type having a frame and a load carrier, comprising a vertically adjustable platform, a pivotable lifting arm which at spaced-apart articulation points is arti- culated to said frame and said platform, respectively, at least one linear motor by means of which the lifting arm is pivotable whilst moving the platform between upper and lower end positions, and a parallel-motion mechanism for maintaining the platform at an essentially constant angle to the horizontal plane when it is being raised or lowered. Background of the Invention Hoists of the above type are long since known and are used for different types of goods vehicles, usually lorries, in order to facilitate loading and unloading. As a rule, such hoists have an operative loading position, in which the platform is extended in an essentially hori¬ zontal direction outside the load carrier, and an inope¬ rative transport position, in which the platform may be either retracted underneath the load carrier or swung up to an essentially vertical position in order to serve as, for instance, an openable door in a superstructure of the vehicle. These prior-art constructions are, in their entire- ty, supported by the vehicle frame or chassis in both the inoperative and the operative position, and all forces and torques acting on the vehicle during loading and unloading are taken up by the chassis. As a result, the chassis has to be correspondingly dimensioned or rein- forced so as to be capable of taking up these forces, which implies an increase in the weight of the vehicle. Furthermore, also the load-bearing structure of the hoist has to be strong and heavy so as to be capable of taking up the forces that arise when the platform and its load are raised or lowered. In vehicles equipped with spring-mounted wheel sus¬ pensions, the transmission of forces and torques from the platform to the chassis causes the load carrier and the platform to be inclined. When, for instance, the hoist is mounted at the rear of the vehicle, the rear spring sus¬ pension is compressed, whereas the front spring suspen¬ sion is extended. The opposite effect, which likewise is disadvantageous, arises when the platform is placed on the ground. For the same reasons, the inclination of the load carrier and the platform changes when the load is moved onto or removed from the platform, which is a serious accident hazard. In an effort to avoid such unde- sirable movements of the chassis, there have been pro¬ vided hydraulic supports, which are lowered from the vehicle onto the ground. DE 27 42 045, DE 10 29 739 and FR 2,140,884, among others, disclose such supports. Owing to the fairly high purchase price and the resulting reduction of effective load due to the dead weight, such supports are, however, mounted on vehicles in exceptional cases only. Prior-art hoists that can be moved in underneath the load carrier require an additional drive means for moving the hoist inwards and outwards, which renders the construction even more expensive as well as heavier. Such prior-art hoists are sensitive to overloading, since they are designed as brackets. The closer to the free outer end of the platform the load is positioned, the more unfavourable the stresses exerted on the hoist construction and the attachment points on the vehicle chassis become. This also restricts the lifting power available during operation. In prior-art vehicle hoists, the parallel-motion mechanism usually consists of two parallel pivot arms which are of equal length and are articulated to the chassis at two bearing articulation points on the chassis and two bearing articulation points on the platform rear edge facing the vehicle. The pairs of bearing articula¬ tions on the chassis and the platform are located in planes that are substantially vertical and parallel. In other words, the platform has to have a rather thick rear edge, which is disadvantageous in many respects. Thus, a platform of considerable thickness is heavy, as well as expensive to manufacture. Also, the upper side of the platform will form a fairly large angle with the under- side, since the front edge of the platform has to be rather thin in order to facilitate the loading and unloading of goods. As a result, the upper side of the platform will slope rather steeply towards the subjacent structure when the platform is placed thereon. The goods is often loaded and unloaded with the aid of hand-drawn pallet loaders. These operations are much more difficult when the platform slopes steeply towards the subjacent structure. Although the rear edge of the platform is rather thick, there will be but a short distance between the bearing articulation points. This not only results in considerable torques necessitating a sturdy structure of the platform parts, but also renders the platform fairly unstable. Also a small play in an articulation or a slight deformation of the rear edge of the platform pro¬ duces a considerable displacement of the free, outer front edge of the platform. In the prior art, a so-called scissors-type lifting table has been provided for moving the platform between a raised and a lowered position. This is achieved owing to the provision of at least two intersecting scissors arms, which are articulated to one another close to their cen¬ tres. At least one articulation of each pair of bearing articulation points provided where the scissors arms are connected to the platform and the frame, consists of a displaceable articulation bearing. In such a scissors- type lifting table, the bearing articulation points of the platform are located in a plane parallel to the plane of the platform, which obviates the inconveniences men¬ tioned above, i.e. the platform can be rather thin, which is advantageous, at the same time as the distance between the bearing articulation points of the platform can be spaced apart by a considerable distance. To provide a vehicle hoist with such a scissors-type lifting table would, however, be disadvantageous in many respects. Among other things, the plane of the bearing articulation points on the chassis has to be parallel to the plane of the bearing articulation points on the platform. Also, the bearing articulation points have to be arranged in pairs opposite to one another, which would make it neces¬ sary for the platform to be supported by a bracket struc- ture projecting outside the load carrier. Furthermore, it should be possible to lower the platform to the extent that it touches the ground or subjacent structure. Since the bearing articulation points on a bracket structure would be located at a certain distance above the ground or subjacent structure, the platform would have to pass the bearing articulation points of the chassis. Further¬ more, such a scissors-type lifting table requires that not only one of the bearing articulation points on the platform but also one of the bearing articulation points on the chassis are in the form of a displaceable articu¬ lation bearing. Summary of the Invention This invention aims at obviating the drawbacks of prior-art vehicle hoists and providing a hoist in which the platform is steadily supported in reliable fashion, despite a thin and, hence, light and less expensive design. At least this aim is achieved by a hoist as set forth in appended claim 1. The invention is based on the insight that, due to the provision of the bearing articulation points of the platform in a plane at least essentially parallel to the plane of the platform, the distance between the bearing articulation points can be much longer than would have been possible with the bearing articulation points arranged at the rear edge of the platform, while at the same time the platform can be of an advantageously thin design. In order to avoid the drawbacks of conventional scissors-type lifting tables of the above type, in which the bearing articulation points on, respectively, the platform and the chassis have to be located opposite to one another in parallel planes, the parallel-motion mechanism according to the invention is equipped with a lifting arm extending between the chassis and the plat¬ form, a lever articulated to the lifting arm, as well as a parallel-motion arm articulated to the lever. As shown in the drawings of embodiments of the invention, the lever may be connected to the platform, while the paral¬ lel-motion arm is connected to the chassis, and vice versa, i.e. the lever may be connected to the chassis, while the parallel-motion arm is connected to the plat¬ form. These two designs of the parallel-motion mechanism fall within the scope of the invention. In the preferred embodiments, the lever is angled, so as to avoid that the parallel-motion mechanism passes a dead-centre position when moving the platform between the upper and lower end positions. A dead-centre position is passed when the two articulation points of the paral¬ lel-motion arm are on a line with the articulation of the lever to the lifting arm. To enable the platform to be raised and lowered, at least one of the bearing articulation points or the arti- culation of the lever to the lifting arm has to be a dis¬ placeable articulation bearing. It is optional which bearing articulation point is a displaceable articulation bearing. For instance, one of the bearing articulation points of the lifting arm, the lever and the parallel- motion arm may be designed as a displaceable articulation bearing. The movement of displacement should be directed towards and away from the other articulation of the same pair. The invention further aims at providing, in accor¬ dance with a preferred embodiment thereof, a hoist in which the chassis, the fixing points thereon and the load-bearing structure of the hoist are not subjected to considerable stresses and, consequently, may be of a weaker, less expensive and lighter design. Another aim of the invention is to provide a hoist which causes less changes of the inclination of the load carrier and the platform during loading and unloading and when moving the load on the platform, without the use of any additional supports reducing the effective load and making the hoist dearer. At least these aims are achieved by a hoist as set forth in the appended subclaims. Yet another aim of the invention is to make the hoist of a preferred embodiment movable to and away from the area beneath the load carrier. Brief Description of the Drawings In the drawings, FIG. 1 is a side view of a preferred, first embodiment of the invention, full lines indicating the plat¬ form in a raised upper position, and dashed lines indicating an intermediate position and a lower position on the ground, FIG. 2 is a side view of the hoist in the lower position with the two-piece platform folded up, FIG. 3 is a side view of the hoist when being moved in underneath the load carrier, FIG. 4 is a side view of the hoist in a completely re¬ tracted position underneath the load carrier, FIG. 5 is a side view of a second embodiment of the hoist according to the invention, FIG. 6 is a side view of a third embodiment of the hoist according to the invention, and FIG. 7 is a view of a forth embodiment of the hoist according to the invention. Description of Preferred Embodiments Preferred embodiments of the invention will now be described in more detail with reference to the accom¬ panying drawings. Thus, Fig. 1 schematically illustrates the rear part of a load carrier generally designated 1. The load carrier may be the platform of a lorry, which also comprises a chassis or frame 2 for supporting the lorry platform. A hoist according to the invention, which is generally designated 3, comprises a platform 4, a lifting arm 5 and a linear motor 6 in the form of a hydraulic cylinder. The lifting arm 5 is, by articula¬ tions 7 and 8, articulated to, respectively, the platform 4 and the chassis 2 via a shifting element 9, which is shiftably arranged on a guide 10 supported by the chassis 2. The piston rod of the hydraulic cylinder 6 is, by a first joint 11, connected to the lifting arm 5 at a point located between the articulations 7, 8. At an outer end 12 separate from the vehicle, the hydraulic cylinder 6 is provided with a supporting plate which, in the position shown in Fig. 1, is directed downwards and applied against the ground. A second joint 13, arranged at the lower end 12, is, via a link arm 14, articulated to the guide 10. When operating the hydraulic cylinder 6, i.e. during the extension or retraction thereof, the platform 4 is raised or lowered between the two end positions shown in the Figure. In order to maintain the platform 4 at a constant angle, preferably in parallel with the horizontal plane, the hoist is fitted with a parallel-motion mechanism which, in the embodiment shown, comprises a parallel- motion arm 15 and an angled lever 16 in addition to the lifting arm 5. The lever 16 is L-shaped and has a long leg 16' and a short leg 16" forming an angle of about 90° with each other. At the point between the two legs, the lever is articulated to the lifting arm 5 by an articu¬ lation 18 and is, at its outer ends, articulated by articulations 19, 20 to, respectively, the platform 4 and one end of the parallel-motion arm 15 which, at its other end, is articulated to the shifting element 9 by an arti¬ culation 21. Since the articulation 7 between the lifting arm 5 and the platform 4 is designed as a displaceable articulation bearing having a hinge pin displaceably arranged in a guide or slot 17 in the platform 4, the latter is movable upwards and downwards, whilst remaining at a constant angle to the horizontal plane. Owing to the geometry of the parallel-motion mechanism illustrated, the articulation 7 will come to occupy its outermost end position when the platform 4 is in an intermediate posi¬ tion, as shown in the Figure, whilst occupying a more retracted position below as well as above this level. As appears from the Figures, the parallel-motion mechanism is unsymmetrical, i.e. the distance between the articulations 8 and 21 is not the same as that between the articulations 18 and 20. This also goes for the dis¬ tance between the articulations 8 and 18, which is not identical with that between the articulations 20 and 21. However, these distances have to be such that the plat¬ form forms the same angle with the horizontal planes during the entire lifting operation. One purpose of such an unsymmetrical parallel-motion mechanism is that the mechanism will not occupy a dead-centre position in any of the platform positions. A dead-centre position is occupied when the articulations 20, 21 of the parallel- motion arm 15 are on a line with the articulation 18 of the lever 16 to the lifting arm 5. In the dead-centre position, tractive and compressive forces in the paral- lel-motion arm 15 do not exert any torque on the articu¬ lation 18 and are thus unable to prevent a change of the position of the platform when loaded. In other words, the platform becomes unstable. The chosen design of the parallel-motion mechanism enables the devising of a vehicle hoist in which the bearing articulation points on the platform are spaced apart and located in a plane that has at least a certain extent in parallel with the plane of the platform, and the bearing articulation points of the mechanism on the chassis are spaced apart and located in a plane that has at least a certain extent in the ver¬ tical direction, no dead-centre position being passed during the displacement of the platform between upper and lower end positions. Figs 2-4 illustrate the function of the hoist when moved to the inoperative transport position beneath the load carrier 1. In a first step, the hoist is lowered onto the ground by retracting the hydraulic cylinder 6. In the preferred embodiment of the invention, the plat¬ form 4 is divided into two parts, which are intercon¬ nected by a hinge (not shown) . As a result, the platform can be folded up by pivoting the rear part of the plat- form over the front part thereof, as shown in Fig. 2. In this position of the platform, a locking means (not shown) provided between the shifting element 9 and the guide 10 can be released so as to permit the shifting element to move along the guide 10. As can be seen in the Figures, the hydraulic cylinder 6 is, in the operative position, slightly inclined relative to a vertical line, such that the lower end of the hydraulic cylinder and the supporting plate 12 are situated slightly farther away from the vehicle than the first joint 11. If the hydrau- lie cylinder is extended by pushing out the piston rod to the position shown in Fig. 2, in which the locking means is released, the hydraulic cylinder will exert a forward- ly-directed force in relation to the vehicle. Since the lower end of the hydraulic cylinder is guided by the link arm 14, the hoist will be raised from the ground while the shifting element 9 is moved in under the lorry plat¬ form, and the joint 13 and the lower end of the hydraulic cylinder are pivoted upwards, as shown in Fig. 3. Final¬ ly, the hoist occupies a completely retracted transport position, as shown in Fig. 4, in which it takes up a minimum of space beneath the load carrier. Preferably, a lock may be arranged to lock the hoist in the transport position. The inventive hoist has a number of advantages over prior-art hoists. For example, the hydraulic cylinder is positioned closer to the point of engagement of the load on the platform and this, combined with the fact that the stresses are transferred directly to the ground, results in that the chassis and the fixing points thereon are subjected to less stress. As a result, the hoist can be of simpler, less expensive and lighter design. Also, the hoist will require less space, both in vertical and lon¬ gitudinal direction, in the inoperative transport posi¬ tion. Consequently, a greater number of vehicle types can be provided with a hoist, including certain types that have had too little space for conventional hoists. Fig. 5 illustrates an alternative embodiment of the invention, which differs from that described above in that the lifting arm 5 is angled and that the point of engagement of the hydraulic cylinder 6 on the lifting arm is located farther ahead in relation to the vehicle. Moreover, the link arm 14 is connected to the shifting element 9, which means that the displacement of the hoist along the guide 10 of the vehicle has to be performed with the aid of a power-generating means (not shown) , preferably a hydraulic cylinder. In this embodiment, the articulation of the lever 16 to the platform 4 consists of a displaceable articulation bearing 19. Also, the outer end portion 22 of the platform 4 is, at least to a certain extent, pivotable downwards about a hinge joint 23 in order to improve the application of the platform against the ground when in lowered position, thus faci¬ litating the loading and unloading of goods. Fig. 6 illustrates a third embodiment of the inven¬ tive hoist in raised as well as lowered position. In this embodiment, the lever 16 is articulated to the frame by a displaceable articulation bearing 19, which travels in the slot 17 formed in the shifting element 9. For this reason, one end of the lever 16 is articulated to the platform by an articulation 21, i.e. it is the other way round compared with the embodiments described above. As is evident from the Figure, the bearing articulation points 8 and 19 on the frame are located in an essential¬ ly horizontal plane, and the displaceable articulation bearing 19 is movable towards and away from the articu¬ lation 8. Fig. 7 illustrates another embodiment of the inven- tion which rather resembles that illustrated in Fig. 6, the platform being shown in raised as well as lowered position. Here, the bearing articulation points 8, 19 of the chassis as well as the bearing articulation points 8, 21 of the platform are, however, fixed, and it is instead the articulation of the lever 16 to the lifting arm 5 that is designed as a displaceable articulation bearing 18. Neither Fig. 6 nor Fig. 7 shows a linear motor for raising and lowering the platform. It is to be understood that the vertical displacement of the platform can be performed in any suitable way, e.g. with the aid of a hydraulic cylinder resting on the ground, as shown in Figs 1-5, or with the aid of a hydraulic cylinder operat¬ ing between the lifting arm 5 and the frame 2. Conceivable Modifications of the Invention It goes without saying that the present invention can be modified in many ways within the scope of the appended claims. Thus, the hoist need not be mounted at the rear of the vehicle but may well be mounted on a long side or even at the front. Also, the means for maintain¬ ing the position of the lower end of the hydraulic cylin¬ der need not necessarily be a link arm 14. For instance, some sort of locking means may be arranged in the joint 11 which, depending on the angular position of the lift- ing arm 5 in the articulation 8, locks the hydraulic cylinder in a fixed position. In a preferred embodiment of the invention, the hoist is provided with two lifting arms 5, two hydraulic cylinders 6, two link arms 14, two parallel-motion arms 15 and two levers 16. However, there is nothing to prevent a hoist design with but one or more than two of either of these. Further, the linear motor must not necessarily be a hydraulic cylinder, but use can also be made of pneumatic cylinders or screws. Also the parallel-motion mechanism may, of course, be designed in many other ways. For example, the lever 16 may have other angles than precisely 90°, and the parallel-motion arm 15 may also be a linear motor, preferably a hydraulic cylinder to permit angular setting of the platform 4 in relation to the horizontal plane. Instead of having the lever articulated to the lifting arm at a central portion and to, respectively, the parallel-motion arm and the platform or the frame at opposite end portions, as in the embodiments shown, one might have the parallel-motion arm connected to a central portion between the articulations to, respectively, the lifting arm and the platform or the frame. In a hoist design where the platform 4 is raised vertically upwards as a door when to occupy the inopera¬ tive transport position, the articulation 7 may advan¬ tageously be located as close as possible to the edge of the platform facing the load carrier. The articulation of the lever 16 to the platform may then be situated outside the platform, i.e. rearwards of the vehicle. In this design, the hydraulic cylinder 6 should be pivotable away from the inoperative transport position. This may be achieved by having a link arm 14 in the form of a hydrau¬ lic cylinder. The joint 13 on the hydraulic cylinder 6 need, of course, not be arranged at the lower end of the cylinder. Further, the hydraulic cylinder 6 need not be connected to the lifting arm in the position shown in the drawings, but the connection may be arranged farther away towards the articulation 7 and may then be provided on the platform 4 itself. The long hydraulic cylinders which are then required may suitably be telescopic in order to have a minimum length in retracted state. The locking means acting on the shifting element 9 may be designed in many ways. For instance, the release and locking thereof may be brought about by pivoting the outer part of the platform in such a manner that the locking means will be released when the platform is folded into the position shown in Fig. 2 and will be locked when the platform is unfolded. It goes without saying that the parallel-motion mechanism described can also be operated with the aid of power-generating means that do not rest on the ground with one end. For instance, the platform might be verti¬ cally adjustable with the aid of a hydraulic cylinder operating between the chassis 2 and the lifting arm 5.

Claims: CLAIMS 1. A hoist for vehicles of the type having a frame (2) and a load carrier (1), comprising a vertically adjustable platform (4), a pivotable lifting arm (5) which at spaced-apart articulation points (7, 8) is arti¬ culated to said frame and said platform, respectively, at least one linear motor (6) by means of which the lifting arm is pivotable whilst moving the platform between upper and lower end positions, and a parallel-motion mechanism (15, 16) for maintaining the platform at an essentially constant angle to the horizontal plane when it is being raised or lowered, c h a r a c t e r i s e d in that the parallel-motion mechanism comprises a lever (16) and a parallel-motion arm (15) which are articulated to each other by an articulation (20) , the lever (16) being arti¬ culated to the lifting arm (5) by an articulation (18) and to the platform (4) or the frame (2) by an articula- tion (19), whilst the parallel-motion arm (15) is articu¬ lated by an articulation (21) to the one of the frame and the platform that is not connected to the lever, such that the parallel-motion mechanism comprises two pairs of bearing articulation points, namely a first pair includ- ing the articulations (7, 19; 7, 21) on the platform and a second pair including the articulations (8, 21; 8, 19) on the frame, said first pair being located in a plane which has at least a certain extent parallel to the plane of the platform, and at least one of the bearing articu- , lation points (7, 19, 8, 21) or the articulation (18) of the lever (16) to the lifting arm (5) consisting of a displaceable articulation bearing. 2. A hoist as set forth in claim 1, c h a r a c ¬ t e r i s e d in that the lever ,(16) is angled. 3. A hoist as set forth in claim 1 or 2, c h a r ¬ a c t e r i s e d in that an intermediate portion of the lever (16) is articulated to the lifting arm (5) , whilst the opposite end portions thereof are articulated to, respectively, the parallel-motion arm (15) and the plat¬ form (4) or the frame (2) . 4. A hoist as set forth in claims 2 and 3, c h a r a c t e r i s e d in that the intermediate portion of the lever (16) is angled. 5. A hoist as set forth in any one of the preceding claims, c h a r a c t e r i s e d in that the displace¬ able articulation bearing is provided adjacent to the articulation of the lifting arm (5) to the platform (4) . 6. A hoist as set forth in any one of the preceding claims, c h a r a c t e r i s e d in that the second pair of bearing articulation points (8, 21) is arranged on a shifting element (9) which is so displaceable along a guide (10) on said frame (2) that the platform (4) is reciprocatingly movable along the guide (10) between two spaced-apart end positions, viz. an inner end position, in which the platform occupies an inoperative position beneath the load carrier (1) of the vehicle, and an outer end position, in which the platform is extended into the area outside the load carrier. 7. A hoist as set forth in any one of the preceding claims, c h a r a c t e r i s e d in that the linear motor (6) is connected by a first joint (11) to the lift- ing arm (5) or the platform (4) and has an outer end (12) separate from the vehicle and placeable on the ground so as to increase or decrease, during operation of the linear motor, the distance between the first joint and said end (12) and, consequently, effect raising or lowering of the platform. 8. A hoist as set forth in any one of the preceding claims, c h a r a c t e r i s e d in that it has means (14) for maintaining the lower end of the linear motor in a constant position relative to the ground, with varying angles in relation to the lifting arm or the platform (4) . 9. A hoist as set forth in claim 8, c h a r a c ¬ t e r i s e d in that said means comprises a link arm (14), whose one end is articulated to the linear motor (6) by a second joint (13) and whose other end is arti- culated to said frame (2) or to the guide (10) . 10. A hoist as set forth in claim 9, c h a r a c ¬ t e r i s e d in that the second joint (13) is located in the area of the lower end of the linear motor (6) .

Inventors: SVENSSON INGEMAR DI, SVENSSON, INGEMAR +DI
Applicants: RAEMSELL BEATRICE & LF, RAEMSELL, BEATRICE +LF
Application year: 1994
Main classification: B60P1
IPC codes: B60P1